DESCRIPTION (The applicant's description verbatim): Acute biventricular (BV) pacing has been shown to result in hemodynamic improvement in patients with left ventricular dysfunction. Based on these studies, the effect of long term BV pacing on exercise tolerance and quality of life are being assessed in large prospective trials. What remains unknown are the effects of BV pacing on sympathetic activity, a known predictor of cardiac mortality, and on the incidence of ventricular arrhythmias. We hypothesize that in patients with LV dysfunction 1) BV pacing decreases sympathetic activity compared to intrinsic conduction in the presence of intraventricular conduction delay 2) BV and LV pacing improves hemodynamics and decreases sympathetic activity compared to right ventricular pacing 3) BV pacing decreases the inducibility of ventricular arrhythmias and 4) that this latter effect is due to preexcitation and prolongation of the coupling interval in the "slow" zone of the tachycardia circuit. To test these hypotheses, consecutive patients referred to the arrhythmia section at the Dallas VAMC with LV dysfunction and an indication for electrophysiologic evaluation will be enrolled in the study. During phase 1 and 2, arterial pressure, central venous pressure and muscle sympathetic nerve activity using microneurography will be recorded during sinus rhythm, atrial pacing (in patients with a QRS greater than 150msw) and atrial-ventricular pacing (RV, LV or BV). Pacing will be performed at a rate 10 beats faster than sinus rhythm. During phase 3 and 4, we will assess the effect of BV pacing on the inducibility of ventricular arrhythmias and the associated electrophysiologic changes. The outcome of this study will have a great impact on our management of patients with congestive heart failure. A reduction in sympathetic activity, demonstrated first with acute BV pacing and later with long term pacing, may have a beneficial effect on mortality. Similarly, a reduction in inducibility of ventricular arrhythmias, if present with long term pacing, may have an impact on the survival and on our management of patients with implantable defibrillators and frequent shocks.